Generally, a sound is a wave propagated in a medium, and is called a sound wave, which is a vibration propagated in the medium. Animals as well as human beings, having an auditory organ, may detect a sound. However, audible frequency ranges of animals and human beings differ, depending on species. An audio frequency, which is a frequency range of audible sound of human beings, is known to be 20 Hz-20,000 Hz.
A typical speaker generates a sound by reciprocally moving a vibration membrane that is called a cone, that is, by moving a vibration membrane back and forth. Taking an example of a woofer that generates a low tone sound, air in front of the vibration membrane is compressed when the vibration membrane moves forward, and air pressure in front of the vibration membrane is lowered by becoming sparse when the vibration membrane moves rearward. That is, when the vibration membrane moves forward, a positive (“+”) sound pressure level occurs in front of the vibration membrane, and when the vibration membrane moves rearward, a negative (“−”) sound pressure level occurs in front of the vibration membrane. A wave occurs by such an operation, and ears of human beings can detect the wave as sound when the frequency of the wave is between 20 Hz and 20,000 Hz.
Generally, an acoustic actuator denotes a speaker or a receiver, among which research is carried out on piezoelectric speakers, for application to portable electronic devices such as a portable terminal and PDA since they are thin and light-weighted in comparison with a conventional VCM (Voice Coil Motor) speaker. A piezoelectric speaker is also called a piezoelectric buzzer, which utilizes a converse piezoelectric effect of a piezoelectric device.
A conventional piezoelectric speaker is formed by attaching a piezoelectric device on a circular vibration membrane made of metal such as brass, stainless steel, a nickel alloy, etc.
A technique for realizing a larger sound pressure level using Helmholtz's resonance principle has been proposed, however, such a technique has been found impractical since an overall exterior diameter of a speaker should be excessively large in order to realize a low frequency of merely about 100 Hz.
As a conventional scheme, an edge of the circular piezoelectric device is fixed, and a hollow space for a resonance is formed in the central area. Such a scheme could amplify only specific frequencies, and did not how a good characteristic at a low frequency or in a wide frequency range. That is, flatness over an entire frequency bandwidth is low, and this means that a sound of a specific frequency may be reproduced but a sound of other frequencies may not be sufficiently reproduced. That means that a sound source having various frequencies may not be reproduced. Thus, such a scheme is used only for a buzzer.
Furthermore, according to the conventional art, the sound pressure level was not high, and thus the resulting sound was too small for a use as an audio speaker.
In an attempt to improve a low frequency characteristic, using a piezoelectric device as a vibration source, a generated vibration is transferred to a panel using a supporting member. However, such a scheme has been found to have drawbacks that a separate panel must be designed, and since the panel is connected with some other component, the vibration is disturbed or the vibration deteriorates electrical or mechanical characteristics of another component.